1. Field of the Invention
The invention relates to a linearly polarized fiber optic laser.
As in all lasers, it may be useful and even necessary in certain applications to lave emission available that is linearly polarized in a stable direction of polarization.
The laser of the invention is therefore a fiber-optic laser that can be used to obtain an emission of a linearly polarized transversal monomode light wave. Such a laser is applicable to a large variety of fields such as telecommunications, optical transmission, instrumentation, spectroscopy, medicine, the detection of chemical species and telemetry.
2. Description of the Prior Art
The optical radiation of a laser emitting a transversal monomode beam is generally constituted by two groups of orthogonally polarized longitudinal modes. In a medium other than a vacuum, these modes have different resonance frequencies. The state of polarization of the emitted wave is random.
In fiber-optic lasers, external stresses such as pressure, vibrations and temperature variations may prompt refractive-index variations and polarization couplings. This may cause the power to vary with time in each polarization so that one polarizer is sufficient to obtain a polarized beam. The power at output of the polarizer may be subjected to random variations even if the total power at output of the laser (before the polarizer) remains constant.
Furthermore, the phenomena of couplings modify the resonance frequencies of the groups of longitudinal modes in an uncontrollable way. This effect proves to be troublesome for all the applications of lasers in which the emission frequency must remain fixed in the course of time.
In certain types of laser, the emission is naturally polarized or can easily be made to be polarized. These are cavities having a differential gain in the two natural modes of polarization. A well-known example of polarized emission is that of semiconductor lasers (GaAlAs) for which the anisotropy of gain due to the structure of the waveguide, associated with the homogeneous character of the transitions brought into play, provides for polarized emission. Another example of artificially polarized emission is that of gas lasers with Brewster plates enclosing the gaseous active element. These plates are designed both to prevent parasitic cavities and to introduce a sufficient differential loss in the two modes of linear polarization (since gas lasers have very low gains, a small difference of loss is sufficient).
The case of lasers doped fiber with rare earths is more particular. First of all, at ambient temperature, the emission lines have a predominant, non-homogeneous component. This permits the simultaneous oscillation of several wavelengths and modes of polarization. Hence the introduction, into these media, of a low differential gain on the natural states of polarization does not suffice to ensure emission rectilinearly polarized along a stable direction (a monopolarization emission actually occurs in the pumping zone ranging between the thresholds corresponding to the two polarizations, but it cannot be exploited in practice). Furthermore, in a conventional fiber, the state of polarization of the laser wave is highly sensitive to the external parameters and may vary rapidly and randomly.